difference between BJT & MOSFET

[sanaprog]

hi everybody.....can anyone one tel me some operational difference between bjt and fet.........hmhm i know bjt is current controlled device AND mosfet is voltage controlled....

---------- Post added at 21:43 ---------- Previous post was at 21:41 ----------

how 2 make the selection that should i use bjt or fet???

 

[WimRFP]

In power electronics, the Silicon BJT is replaced by the MOSFET and IGBT.

At low voltage (say below 200V), mosfet has low on-state voltage drop and is fast. The BJT may have even lower on-state voltage drop at higher voltage, but has poor off-switching characteristic because of the storage time, so switching loss will be higher than in MOSFET. Also you have to apply current to keep it in the on-state. At >200V, BJT has lowest on-state voltage, but HV Silicon BJT have low gain, so you need lots of base current.

At voltage > 300V, IGBT becomes attractive because of the on-state voltage drop and similar drive requriements as for a mosfet: you only need to charge and discharge the parasitic capacitances. At high switching frequency, the MOSFET will win, despite the higher on-state loss. The reason is the better switching speed of mosfet over IGBT. You may know that IGBT gets its high current density because of BJT operation). The world may change with the advent of wide band gap material based components (for example SiC).

In small signal linear applications, the BJT is very popular, it is cheap, has high transconductance at low current and can be operated at low voltage and is a normally-off device. The JFET is normally-on, so you need negative gate voltage (or source resisitor) in many applications.

In very low voltage applications (say mV range), you can't use BJT as you don't have sufficient voltage to bias the BE junction, a field effect device can be normally on and can still provide power gain at mV supply voltage.

In systems that require very high input impedance, mostly a FET is used because of it is a voltage controlled rather then a current controlled device.